a photographic edge-isolation technique€¦ · paper describing the application of the same...
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393
ALVA B. CLARKE,
U. S. Geological Survey, Washington, D. C.
ALVA B. CLARKE
some degree, all of the edges as they appear inthe aerial negative and in precisely the samelocation. Basically, at present, there are twomeans which may be used for edge enhancement: (1) electronic image-intensifier systems, and (2) direct photographic processes.It is not wi thi n the scope of this paper to discuss or eva1uate systems other than the onespecifically described here in.
Photographic processes of edge enhancement are simple and readily applicable totopographic mapping. One method of gaining"edge-contrast" by use of a film maskingtechnique, was described by Messers. Yule l
and Clerc' in 1945 and 1955 respectively. Apaper describing the application of the samemasking technique to aerial photographs waspresented by Messrs. John St. Clair andMylon Merriam in 1960.3 Results achievedby this method are promising.
1 Yule, J. A. c., Journal of Photographic Societyof America, Vol. 11:123, 1945.
2 Clerc, L. P., Photography, Theory and Practice,3rd ed., London. 1955, p. 374.
3 St. Clair, John and Merriam,. Myl.on~ "TheTo emphasize the detail of maximum sig- Use of Photography to Achieve BasIc Pnnclples of
nificance while subduing detail of lesser im- Map Design," PHOTOGRAMMETRIC ENGINEERING,portance, it is necessary to emphasize, in June 1960. Vol. xxvi, no. 2, p. 498.
* Presented at 28th Annual Meeting of the American Society of Photogrammetry, March 16, 1962.Publication approved by Director, U. S. Geological Survey.
1: Abstract is on page 349 of 1962 YEARBOOK.
EDGE ENHANCEMENT
A Photographic Edge-Isolation Technique*!
T HE oldest of all art forms is the representation of objects and ideas by lines. A
capable artist can usually project his thoughtby using a minimum of lines. The idea, then,of representing real objects by using edgesformed during a photographic process is notinconceivable. The primary difference between photographic representation and artistic rendition (Figure 1) is that the formerfulfills one of the basic requirements of mapping, that of measurability, while the otherdoes not.
The process of mapping from aerial photographs includes observing and interpretingpertinent information contained in an aerialnegative. Photographic film acts as a memoryunit, recording information transferred to itby an optical system. Differences in luminance due to color variation, sun angle, andshadows of the terrain and objects upon it arerecorded in their relative positions as differences in density. If the unit distance on thefilm is small, the density differences aresharply defined and are termed "edges."These edges reveal much of the informationthe interpreter needs to locate and identifyimages on the photograph. For some purposes and under certain circumstances, edgesalone, without intervening tonal variations,can convey sufficient information.
I n aerial photographs, imagery of maximum significance to the interpreter or photogrammetrist is characterized by the size,shape, and pattern of the edges formed duringthe photographic process. Unbroken, straight,or smooth continuous curved lines usuallyindicate man-made or man-modified objects.Continuous but irregularly curved lines indicate natural features, such as drainage systems. Closed erratic patterns generally indicate vegetation. ]\'ote that all of these features have a common characteristic-theycan be represented by a single line describingtheir edges.
FIG. 1. Comparison of a photograph (Left), a photographically outlined subject (Center), and anartists' free hand drawing (Right).
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A PHOTOGRAPHIC EDGE-ISOLATION TECHNIQUE 395
EDGE ISOLATION
In 1959, a technique was tested in theTopographic Division of the Geological Survey whereby the compression of density inan aerial photograph could reach a nearmaximum while preserving and in many casesincreasi ng the con trast of fine detail. I twastheorized that to completely eliminate tonesof lesser importance, more control over contrast was required. The ensuing process wascalled "edge isolation," and the first testsindicated that it was adaptable to negativesof extreme density which are beyond thenormal range of automatic-dodging printers.
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• Watson, Alfred J., The Fluoro-Dodge methodfor contrast ("ontral. PHOTOGRAMMETRIC ENGINEERING, September 1958. Vol. xxiv, no. 4, p. 638.
PRIKCIPLES
Other than film, the only equipment necessary for the edge-isolation process is a specialcontact printer manufactured under the tradename "Fluoro-Dodge."4 The operation ofthis printer is based on photo-luminescence,which is the property of certain substances toabsorb radiation of one wave-length and emitradiation of another wave-length. The photoluminescent substance used in this system iszinc sulfide. In conjunction with its photoluminescent property or, more specifically,phosphorescence due to ultraviolet absorp-
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BORDER EFFECTS CAUSED
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tion, zinc sulfide is also inversely sensitive tored and infrared. In other words, red andinfrared radiation suppresses the luminescence of the zinc sulfide phosphor, as shownin Figure 2.
The components of the Fluoro-Dodge system are outlined in the schematic diagram
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EFFECTS OF INFRARED ON FLUORESCENCE
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396 PHOTOGRAMMETRIC ENGINEERING
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-------------0FIG. 6. Transition from a density step to a line.
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shown in Figure 3. The lamp (UVL) radiatesa range of ultraviolet rays with a peak emission at 3650 Angstrom units, cxciting the
phosphor screeen (P), which luminesces. Thefilter (UVF) absorbs the ultraviolet raysshorter than 4000 Angstrom units and transmits visible light. The filter (IRF) transmitsinfrared above the visible range, emanatingfrom the incandescent lamps (IL), quenchingluminescence from the phosphor screen (P).The quenching action of the infrared througha negative (N) converts the phosphor screeninto a point-to-point image-modulated lightsource.
FORMATION OF A LINE
A determining factor in the formation of aline of finite width by the edge-isolation process is the distance between the photographicnegative and the phosphor screen. Infrareddiffusion from the areas of less density acrossthe edges into areas of greater density,emphasizes the border. The effect produced issi milar to the border effects of normal developmen t (Figure 4). Figure 5 is a microtrace across seven consecu ti ve steps of astandard density step wedge after compression has taken place. By manipulation of adeveloper and exposure, it is possible toobtain any desired degree of masking on thefinal negative.
Proper selection of film and developer combinations enables the operator to control thedegree of edge emphasis. For this application,
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FIG. 7. Stone Mountain, near Atlanta, Georgia. Large clear area, upper center, is in dense shadowformed by a high cliff. Xote the pattern on the opposite side of the mountain, indicating drainage systemsleading to the small lakes and ponds in the wooded area at lower levels.
FIG. 8. Semirural area in Southern California. Here the difference between the lines formed by naturalvegetation and those formed by manmade objects is very apparent. Areas in both figures 7 and 8 were usedin a study in contrast control presented to the Society in 1961 (Contrast Control for Diapositives by JamesC. Lewis).
398 PHOTOGRAMMETRIC ENGINEERING
FIG. 9. Part of Alexandria, Virginia, much of the culture shown on this photograph is of recentorigin and does not appear on published maps.
FIG. 10. Alexandria, Virginia, area at smaller scale. The first step of this edge-isolated photographwas made with Kodak auto-screen ortho. The edge effect is very pronounced, without total loss of intermediate tone.
A PHOTOGRAPHIC EDGE-ISOLATION TECHNIQUE 399
the edges should be completely isolated fromintermediate tones and rendered as clear linesagainst an opaque background. Figure 6illustrates the transition from a simple densitydifference to total edge isolation.
PROCEDURE
The emulsion for this transformation musthave extremely high density and contrastcapabilities and must be coated on a stablebase. In the first step, a positive transparencyis made on high-contrast orthochromatic filmfrom the original negative, with maximumquenching and developed in a moderatelycontrasty developer, such as Kodak D-19.The dark lines along the image borders, asdescribed above, will have a density higherthan that of the surrounding background.The clear lines adjacent to them allow additional passage of infrared on the succeedingstep and thus further accentuate the darklines, which are then printed in negativeform.
The final negative is printed in the sameway as the positive transparency. After theinitial exposure, however, the exposed negative remains in the printer for a short time toallow the afterglow of the phosphor screen toexpose the film in the background areas. Theextent of edge isolation depends on the characteristics of the developer used for thisnegative. For complete isolation or totalelimination of tone, the film is processed in acaustic developer, such as Kodalith. The
background becomes opaque, and only clearlines are left to represent the original imageedges.
The accompanying slides demonstrate thetype of line which can be expected. They are,of course, greatly magnified and show somedegradation (Figures 7, 8, 9,10).
The edge-isolation process offers someattractive possibilities for saving time andmoney, especially in producing special-purpose maps for which standard cartographicsymbolization is not necessary. Currently, allmapworthy features must be represented bygraphically constructed symbols, manuallylocated. The photographic processing of imagesymbols by edge isolation provides a semiautomatic method of representing prominentterrain features and in many respects, thesymbolization is superior. When made fromorthophotographs, the imagery is accuratelypositioned and the resulting product is a veryuseful planimetric map. In addition, edgerepresented maps do not require halftonescreening for reproduction by lithography.Patterns val uable to geologists and othersinterested in the general configuration of theterrain are often more easily detected in edgeisolated prints than in the original photograph.
The relative simplicity of processing edgeisolated prints could result in their widespread use. It is, however, impossible topredict at this time all the benefits which mayresult from this process.
AP/ i-A New Concept zn Stereoplotting*
S. JACK FRIEDMAN,
OMI Corporation of America,Washington, D. C.
I N PREPARING this paper the original intention was to briefly review the principles of
the Analytical Stereoplotter and then to givesome detailed information on its design andconstruction. And finally to discuss brieflythe uses to which the instrument could be put.I found, however, that during the past yearor two in which we have been developing theprototype this information has already beenmade available in several papers. Mr. Helava,
the inventor of the instrument itself, has presented some excellent material on the principles as well as the contemplated use of suchinstrument and now I notice in a recent issue of "PHOTOGRAMMETRIC ENGINEERING,"that Dr. Johnson of the Bendix ResearchLaboratories Division has presented a verythorough discussion of the basic concepts involved in the prototype instrument.
Also I must confess that I myself have been
* Paper presented at the Semi-Annual Meeting of the American Society of Photogrammetry, October1961, New York.